The present invention relates to a stepwise process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps by the action of oxygen, which additionally comprises a subsequent bleaching treatment with peroxide.
The bleaching of chemical and semi-chemical cellulosic pulps intended in particular for the manufacture of paper, in general requires several successive bleaching treatments, optionally with the interposition of washing, dilution and/or concentration stages, in order to arrive at the desired residual lignin content and the desired whiteness.
Various types of bleaching agents are commonly used for the treatment of cellulosic pulps. Thus it has been proposed to subject chemical and semi-chemical pulps to the action of oxygen or of air in an alkaline medium and then to bleaching treatments comprising a treatment with peroxide. However, the removal of the lignin, and the increase in whiteness, are accompanied by substantial changes in the physical characteristics of the cellulosic pulps and more particularly by a significant reduction in the viscosity of the pulps, revealing a depolymerisation of the cellulose chains. Operated on an industrial scale, the yield of the processes is low. Furthermore, the strength of the final products is diminished.